The present invention relates to a novel cooling system for increasing the useful life of a turbine bucket.
A gas turbine has (i) a compressor section for producing compressed air, (ii) a combustion section for heating a first portion of said compressed air, thereby producing a hot compressed gas, and (iii) a turbine section having a rotor disposed therein for expanding the hot compressed gas. The rotor is comprised of a plurality of circumferentially disposed turbine buckets.
Referring to FIG. 1, each turbine bucket 10 is comprised of an airfoil portion 12 having a suction surface and a pressure surface; and a root portion 14 having structure 18 to affixing the blade to the rotor shaft, a platform 16 from which said airfoil extends, and a shank portion 20.
The platforms are employed on turbine buckets to form the inner flow path boundary through the hot gas path section of the gas turbine. Design conditions, that is gas path temperatures and mechanical loads, often create considerable difficulty to have bucket platforms last the desired amount of time in the engine. In this regard, the loading created by gas turbine buckets create highly stressed regions of the bucket platform that, when coupled with the elevated temperatures, may fail prior to the desired design life.
A variety of previous platform cooling designs have been used or disclosed. Referring to FIG. 2, one previous platform cooling design was based on utilizing the cavity 122 formed by adjacent bucket shanks 120 and platforms 116 as an integral part of the cooling circuit. This type of design extracts air from one of the buckets internal cooling passages and uses it to pressurize the cavity 122 formed by the adjacent bucket shanks 120 and platforms 116 described above. Once pressurized, this cavity can then supply cooling to almost any location on the platform. Impingement cooling is often incorporated in this type of design to enhance heat transfer. The cooling air may exit the cavity through film cooling holes in the platform or through axial cooling holes which then direct the air out of the shank cavity. This design, however, has several disadvantages. First, the cooling circuit is not self contained in one part and is only formed once at least two buckets 110 are assembled in close proximity. This adds a great degree of difficulty to pre-installation flow testing. A second disadvantage is the integrity of the cavity 122 formed between adjacent buckets 110 is dependent on how well the perimeter of the cavity is sealed. Inadequate sealing may result in inadequate platform cooling and wasted cooling air.
Another prior art design is disclosed in FIGS. 1(a) and 5(a) of U.S. Pat. No. 6,190,130. This design uses a cooling circuit that is contained fully within a single bucket. With this design, cooling air is extracted from an airfoil leading edge cooling passage and directed aft through the platform. The cooling air exits through exit holes in the aft portion of the bucket platform or into the slash-face cavity between adjacent bucket platforms. This design has an advantage over that described above and depicted in FIG. 2 in that it is not affected by variations in assembly conditions. However, as illustrated therein, only a single circuit is provided on each side of the airfoil and, thus, there is the disadvantage of having limited control the amount of cooling air used at different locations in the platform. This design also has the disadvantage of restricting the cooling air supply to the leading edge cavity.
Yet another prior art cooling circuit configuration is disclosed in FIG. 3(a) of U.S. Pat. No. 6,190,130 and also in U.S. Pat. No. 5,639,216. This design also uses a cooling circuit fully contained within a single bucket, but it is supplied by air from underneath the platform, i.e. shank pocket cavity or forward wheel space (disc cavity).